Intense surface currents in the tropical Pacific during 1996–1998

Abstract

Tropical Pacific Ocean near‐surface currents and their momentum and temperature balances are investigated using several data sets, including drifting buoy velocities and TOPEX/Poseidon altimetry. The data sets are combined to produce monthly surface current estimates on a uniform grid for the 6‐year period 1993–1998, using multivariate optimal interpolation. The analysis shows dramatic changes in current from December 1996 through August 1998 in response to the recent El Niño‐Southern Oscillation event. Eastward current anomalies of ∼1 m s−1 were recorded in December 1996 in the western Pacific, generated by sporadic westerly wind bursts. By April 1997 these equatorial anomalies reached the eastern boundary, and in the summer of 1997 a band of strong eastward flow formed across the basin. This circulation pattern persisted until the beginning of 1998, when a westward equatorial jet appeared in January‐April. Interestingly, the reversal of flow occurred prior to the restoration of the trade winds. The timing of these events as well as the results of previous dynamical studies raise questions about the relative importance of terms in the zonal momentum and temperature balances. We address the first of these questions by examining the applicability of a three‐term linear zonal momentum balance on the equator. In all cases we focus on anomalies from the time mean. Our results show that local acceleration is reasonably well balanced by the difference between zonal pressure gradient anomaly and wind‐induced momentum flux. This three‐term balance displays significant basin‐wide variations and is consistent with the conclusion that the equatorial Pacific is not in equilibrium with local wind forcing due to the presence of propagating waves. Examination of the temperature balance shows that at the beginning of El Niño the warming in the central Pacific is mainly supported by horizontal temperature transport, while the vertical heat exchange and transport become important in the eastern half of the equatorial Pacific during the transition from El Niño to La Niña.